1. Field of the Invention
The present invention is directed to a CRT display device using a cathode-ray tube.
2. Description of the Background Art
FIG. 4 is a block diagram schematically illustrating a CRT display device conventionally used in the background art. The reference numeral 1 represents a CRT. As shown in FIG. 4, the CRT 1 has an electron gun including a cathode 2 for emitting electrons, a first electrode G1, a second electrode G2, a third electrode G3, an anode 3 and the like. The first electrode G1, the second electrode G2 and the third electrode G3 of a cylindrical shape respectively having electron passing holes are arranged in the electron gun for drawing electrons from the cathode 2 and performing prefocusing, for example. The elements such as a focus electrode to be arranged subsequently to the third electrode G3 are omitted from FIG. 4 for the convenience of description. A flyback transformer (FBT) and a variable resistor are designated by the reference numerals 4 and 5, respectively. A video cathode amplifier, a cathode bias voltage source which generates a voltage for drawing electrons from the cathode 2, an adjusting/input circuit and a video signal to be inputted to the CRT display device are designated by the reference numerals 6, 7, 8 and 9, respectively.
Among voltages to be inputted from the cathode bias voltage source 7 to the cathode 2, a voltage for permitting a current to start flowing from the cathode 2 is called as a cutoff voltage (bias voltage for display in black). Considering the cutoff voltage to be 0 V, a voltage further inputted to the cathode 2 is called as a driving voltage.
The CRT display device is generally provided with brightness adjustment function and contrast adjustment function. The brightness adjustment function is intended, according to user""s needs, to adjust a black level of an image and a level at which a screen starts to emit light. Normally, the level of the bias voltage for display in black, namely, the level of the cutoff voltage of the signal defined by the video signal 9 and to be applied to the cathode 2, is changed to perform the brightness adjustment function. The change in level of the cutoff voltage is controlled by adjusting potential difference between the cathode 2 and the first electrode G1. The adjusting/input circuit 8, intended to control the cutoff voltage, is composed of a volume resistor, a D/A converter and a microcomputer, for example.
The contrast adjustment function is intended to adjust, according to the user""s needs, the ratio in brightness between a darkest portion and a brightest portion of a screen. Normally, the amplitude of the signal defined by the video signal 9 and to be applied to the cathode 2, more particularly, the driving voltage generated from the cathode bias voltage source 7, is changed to perform the contrast adjustment function.
Next, the operations of the CRT display device will be described in reference to FIG. 4. The video cathode amplifier 6 amplifies the video signal 9. The amplified video signal 9 is converted into a cathode bias signal according to the cutoff voltage controlled by the adjusting/input circuit 8. The cathode bias voltage source 7 generates the driving voltage on the basis of the cathode bias signal and applies the same to the cathode 2.
The flyback transformer 4 boosts and rectifies a horizontal flyback pulse generated at a horizontal deflection output circuit (not shown) to generate a high voltage of about 25 kV which is supplied to the anode 3. The high voltage generated by the flyback transformer 4 is divided by the variable resistor 5 having a resistance value of about 100 Mxcexa9 and supplied to the second electrode G2. The voltage applied to the second electrode G2 is controlled to thereby perform coarse adjustment of a point where emission of light starts by the electrons flowing into the anode 3 and falling on the screen, that is, adjustment of the screen. The voltages of the first electrode G1, the second electrode G2 and the third electrode G3 are respectively set to be 0 V, 500 V and 5.5 kV, for example.
The brightness of the screen of the CRT 1 is defined to be proportional to the value of a current falling on the screen, namely, to the amount of electron beams drawn from the cathode 2 and reaching the screen. That is, the value of the current drawn from the cathode 2 is large in a high brightness condition and small in a low brightness condition.
FIG. 5 is a graph showing the relation between the driving voltage and the value of the current drawn from the cathode 2 by the driving voltage in the CRT display device in the background art. A horizontal axis shows the driving voltage to be inputted to the cathode 2 and a vertical axis shows the value of the current drawn from the cathode 2. As shown in FIG. 5, when the driving voltage is lowered, the value of the current drawn from the cathode 2 is decreased. When the driving voltage is raised, the value of the current drawn from the cathode 2 is increased. That is, the brightness of the screen of the CRT 1 is increased accompanied by the raise in driving voltage.
In recent years, a CRT display device has been desired to show improvement in brightness and resolution. Due to this, development of a multimedia tube (hereinafter referred to as MM tube) as a CRT having high brightness and high resolution has been desired. According to a triode structure of the CRT in the background art as described above, the driving voltage is raised to amplify the value of the current drawn from the cathode 2. Accompanied by the necessity of improvement in resolution of the CRT, however, a frequency of the signal Voltage defined by the video signal 9 to be inputted to the cathode 2, namely, a frequency of the driving voltage, is required to be very high. For this reason, capabilities of the video cathode amplifier 6 and the cathode bias voltage source 7 for generating the driving voltage are approaching their limits. The output of the driving voltage generated by the existing video cathode amplifier 6 and the cathode bias voltage source 7 is limited to be 45 V. It is difficult to obtain a driving voltage exceeding 45 V. In view of these circumstances, it results that the effort to obtain high brightness of the CRT display device in the background art by increasing the driving voltage is approaching its limit.
It may be possible to decrease voltages of the second and third electrodes G2, G3 and lower a cathode voltage during cutoff to obtain high brightness. However, this results in an increased spot diameter on the screen and deterioration in resolution.
As a countermeasure for these problems, a CRT display device has been suggested which suppresses increase in spot diameter on the screen and deterioration in an image quality while allowing electron rays of the amount approximately the same as the background-art CRT display device to be drawn by a driving voltage smaller than that in the background-art CRT display. device, to thereby obtain brightness of a level that is not allowed by the driving voltage in the background art.
Such CRT display device allows high brightness. On the other hand, it has also a disadvantage in that accompanied by improvement in sensitivity of an output current of the CRT to a driving voltage, noises of an image not conspicuously recognized during display with low gradation level in the background-art CRT display device are increased and become conspicuous.
A first aspect of the present invention is directed to a CRT display device, comprising: an electron gun having a cathode and first, second electrodes respectively including electron passing holes for drawing electrons from the cathode; and driving voltage generating means for generating a driving voltage for drawing electrons from the cathode, wherein a cathode voltage during cutoff ranges from 50 V to 80 V regarding a voltage of the first electrode as zero following conditions among a diameter of the electron passing hole of the first electrode, a thickness of the first electrode at the electron passing hole, a diameter of the electron passing hole of the second electrode, a thickness of the second electrode at the electron passing hole and a distance between the first, second electrodes are satisfied,
thickness of first electrode at electron passing hole/diameter of electron passing hole of first electrode xe2x89xa60.23;
distance between first, second electrodes/diameter of electron passing hole of second electrode xe2x89xa60.53; and
thickness of second electrode at electron passing hole/diameter of electron passing hole of second electrode xe2x89xa60.87,
and the driving voltage in an area having a gradation level lower than a predetermined gradation level is kept low in order to keep brightness in the area of the gradation level lower than the predetermined gradation level lower than real brightness of a first video signal to be inputted to the CRT display device.
A second aspect of the present invention is directed to the CRT display device according to the first aspect, wherein the driving voltage is interpolated so that rapid change in voltage level at a boundary between the area of the gradation level lower than the predetermined gradation level and an area of a gradation level higher than the predetermined gradation level is suppressed.
A third aspect of the present invention is directed to the CRT display device according to the first aspect, further comprising signal converting means for converting the first video signal into a second video signal having a gradation level lower than a gradation level of the first video signal in the area of the gradation level lower than the predetermined gradation level, wherein the driving voltage generating means generates the driving voltage on the basis of the second video signal.
A fourth aspect of the present invention is directed to the CRT display device according to the third aspect, wherein the second video signal is interpolated so that rapid change in gradation level at a boundary between the area of the gradation level lower than the predetermined gradation level and an area of a gradation level higher than the predetermined gradation level is suppressed.
According to the CRT display device of the first aspect of the present invention, the driving voltage in the area of the gradation level lower than the predetermined gradation level is kept low in order to keep brightness in the area of the gradation level lower than the predetermined gradation level lower than the real brightness of the first video signal to be inputted to the CRT display device. As a result, noises of an image at a low brightness area conspicuously recognized in the CRT display device using the MM tube in the background art become inconspicuous. Further, as the difference in brightness level between the low brightness area and a high brightness area is enlarged, there occurs increase in contrast value and improvement in image quality.
According to the CRT display device of the second aspect of the present invention, in the CRT display device of the first aspect, the driving voltage is interpolated so that rapid change in voltage level at the boundary between the area of the gradation level lower than the predetermined gradation level and the area of the gradation level higher than the predetermined gradation level is suppressed. As a result, display of an image on the CRT is realized with a desired gradation level in the vicinity of this boundary and deterioration in image caused by the display with undesired gradation level is controlled.
According to the CRT display device of the third aspect of the present invention, in the CRT display device of the first aspect, the signal converting means is provided for converting the first video signal into the second video signal having a gradation level lower than the gradation level of the first video signal in the area of the gradation level lower than the predetermined gradation level and the driving voltage generating means generates the driving voltage on the basis of the second video signal. As a result, the driving voltage in the area of the gradation level lower than the predetermined gradation level is kept low and brightness in the area of the gradation level lower than the predetermined gradation level is kept lower than the real brightness of the first video signal to be inputted to the CRT display device. Therefore, noises of the image in the low brightness area conspicuously recognized in the CRT display device using the MM tube in the background art become inconspicuous. Further, as the difference in brightness level between the low brightness area and the high brightness area is enlarged, there occurs increase in contrast value and improvement in image quality.
According to the CRT display device of the fourth aspect of the present invention, in the CRT display device of the third aspect, the second video signal is interpolated so that rapid change in gradation level at the boundary between the area of the gradation level lower than the predetermined gradation level and the area of the gradation level higher than the predetermined gradation level is suppressed. As a result, rapid change in level of the driving voltage at this boundary is controlled, to thereby suppress rapid change in image at this boundary to be displayed on the CRT. Therefore, display of the image on the CRT is realized with a desired gradation level in the vicinity of this boundary and deterioration in image caused by the display with undesired gradation level is controlled.
It is therefore an object of the present invention to provide a CRT display device allowing electron rays of the amount approximately the same as the background-art CRT display device to be drawn by a driving voltage smaller than that in the background-art CRT display device, to thereby obtain brightness of a level that is not allowed by the driving voltage in the background art. It is also an object of the present invention to allow the CRT display device to be operative to prevent conspicuous recognition of noises and to improve contrast.